1. Field of the Invention
The present invention relates to an actuator of a passenger protecting system for a vehicle.
2. Description of the Related Art
In a conventional vehicle passenger protecting system, an air bag is inflated at a certain time after a belt pre-tensioner is actuated or the air bag is inflated by first and second squibs through two consecutive steps. For example, the second squib of the air bag is actuated to inflate the air bag completely about 50 milli-seconds after the first squib of the air bag is actuated to inflate the air bag initially. If a mechanical switch is used for actuating the first squib, it is difficult to assure the 50 milli-second period.
U.S. Pat. No. 5,657,831 (or JP-A-9-20205) proposes an actuator composed of a microcomputer, three electronic switches, two acceleration sensors and an integration circuit for the above purpose. In the actuator, when a microcomputer judges a collision of a vehicle based on a signal from a first acceleration sensor, the microcomputer turns on a first electronic switch. When a second acceleration switch is turned on, the integration circuit starts to integrate the signal voltage from the second acceleration sensor, and provides a signal to close the second electronic switch if the integrated signal voltage achieves a prescribed level.
However, it is not possible to diagnose a malfunction of the second electronic switch because the integration circuit is separated from the microcomputer. In addition, because all three switches are electronic switches, electrical noise, as well as moisture, may compromise the operation of the actuator.
JP-A-9-2196 proposes another actuator in which a switch is closed by a signal from a mechanical acceleration sensor. This actuator has substantially the same problem as the above.